Structural and Dynamical Roles of Bound Polymer Chains in Rubber Reinforcement

Abstract

The addition of nanofillers to rubber matrices is a powerful route to improve the mechanical properties. Here, we focus on a molecular understanding of basic mechanisms that are important for the reinforcement in rubbers. The key role in this process is ascribed to bound rubber (BR) that engages with the matrix as well as with adjacent nanofillers. To date, this understanding has been impeded by the lack of experimental tools to directly probe the BR chains buried in a polymer matrix composed of the same polymer. To tackle this challenge, we combine neutron scattering/spectroscopy techniques with isotope-labeling and molecular dynamics simulations. The system is a simplified carbon-black-filled polybutadiene. The combined experimental and computational results provide new insights into the local structural and dynamical heterogeneities of BR chains and their interactions with the matrix polymer, highlighting (i) the structural partition of the bound chains into three components (i.e., trains, loops, and tails) and their fractions; (ii) their dynamical hierarchies, i.e., the trains that remain immobile on the filler surface, the loops that are fairly large and hence allow the interdigitation of matrix chains, and the tails with their unique characteristics to reach far out into the matrix and entangle with matrix chains. These multiple roles of the constituent components of the BR chains promote the formation of a well-developed adhesive polymer−filler interface, enhancing the elastic property of a filled rubber. The comprehensive understanding derived and validated by the model rubber will be translatable to many other polymer nanocomposites.

Thermally Activated Shear Stiffening in Polymer-Grafted Nanoparticle Composites for High-Temperature Adhesives

“…Interfacial chain mobility gradient, chain relaxations, and particle dynamics have been extensively explored to understand the dynamic response of polymer nanocomposites. 9−17 The viscoelastic properties of composites consisting of one homopolymer type are governed by chain conformations, 18,19 adsorption, 18,20,21 and confinement of chains 22−25 in the vicinity of nanoparticles.…”

“…Rheological studies have largely investigated the role of polymer bridging, , bound polymer chains, particle dispersion, − and particle loading , on reinforcement of nanocomposites. Interfacial chain mobility gradient, chain relaxations, and particle dynamics have been extensively explored to understand the dynamic response of polymer nanocomposites. − The viscoelastic properties of composites consisting of one homopolymer type are governed by chain conformations, , adsorption, ,, and confinement of chains − in the vicinity of nanoparticles.…”

Recent scattering approaches to structure and dynamics of polymer nanocomposites

“…The results for both weak and strong polymer-nanoparticle interactions were found to scale simply with the ratio of bottleneck size between NPs and radius of gyration (Gam et al, 2011). Based on the highresolution NSE and BS spectroscopies complemented with coarse-grained simulations, Salatto et al (2021) investigated the dynamics of the bound rubber layer in carbon black PNCs in detail. The results indicate the existence of entanglements and interdigitation of the bound rubber chains with neighboring free chains in the matrix, leading to a well-developed and adhesive interphase (Figure 2).…”

Quantifying structure and dynamics of bound and bulk polymer in tailor-made rubber-silica nanocomposites

“…Recently, very valuable efforts to relate the so-called bound rubber to the reinforcement were undertaken by the group of Koga [23]. Using neutron scattering and molecular dynamics simulations in the melt, the interphase between polybutadiene (PB) and carbon black was studied.…”

“…It is well known that the unsaturated double bonds of polybutadiene interact strongly via π-π interactions with the graphitic-like surface of carbon black. Salatto et al [23] were able to discriminate between a totally immobilized rubber component of about 1 nm thickness and a more disordered fraction of loops, end-and multilinked chains. The dynamics observed via neutron spin echo spectroscopy was assigned to the latter fraction.…”